Process control systems, like those used in chemical and petroleum processes, typically include one or more process controllers communicatively coupled to at least one host or operator workstation and to one or more field devices via analog, digital, or combined analog/digital buses. The field devices, which may be, for example, valves, valve positioners, switches, and transmitters (e.g., temperature, pressure, and flow rate sensors), perform functions within the process plant, such as opening or closing valves and measuring process parameters. The process controllers receive signals of process measurements made by the field devices and/or other information pertaining to the field devices; use this information to implement control routines; and subsequently generate control signals that are sent over the buses to control the operation of the field devices. With the information collected from the field devices and process controllers, an operator or a technician can execute one or more applications at an operator workstation that perform any desired function with respect to the process, such as, for example, configuring the process, viewing the current state of the process, and/or modifying the operation of the process.
In many cases, field devices may require on-site setup, configuration, testing, and maintenance. For example, before a field device can be installed at a particular location at a process control plant, the field device may need to be programmed and may then need to be tested before and after the field device is installed. Field devices that are already installed may also need to be regularly checked for maintenance reasons or, for example, when a fault is detected and the field device needs to be diagnosed for service or repair. Generally, configuration and testing of field devices are performed on location using a handheld, portable maintenance tool. In fact, because many field devices are installed in remote, hard-to-reach locations, it is more convenient for a user to test the installed devices in such remote locations using a handheld, portable tool rather than using a full configuration and testing device, which can be heavy, bulky, and non-portable, generally requiring the installed field device to be transported to the site of the diagnostic device.
In the case where a field device is at least partially operational and supplied with power, a handheld maintenance tool or portable testing device (“PTD”) can connect to a communication terminal of the field device to run a diagnostic routine. Generally, the field device and the PTD communicate over a two-wire connection. For example, a FOUNDATION® Fieldbus device uses a two-wire connection for testing and communicating with the handheld device.
In some cases, testing a field device on location may not be possible unless power is supplied to the field device. This complication occurs, for example, when there is a power outage, when there is a power issue localized to the field device itself, or when one or more field devices are offline, i.e., in fault situations. Generally, power may be provided to the field device by connecting the field device to a power source via a two-wire power line. For example, FOUNDATION® Fieldbus devices are powered via the same terminals used for communicating with the fieldbus device. However, portable power considerations and Intrinsic Safety (“IS”) standards restrict the manner in which power can be provided to a field device, especially when the field device is installed in a critical or dangerous process control system in the field.
Generally, higher voltages are used for providing power to the field device than voltages used for communicating with the field device. Additionally, certain safety measures must be implemented before powering a field device in the field. In particular, according to IS guidelines, a technician cannot switch on the power of a field device within the field device itself. Accordingly, the technician servicing the field device cannot use or install a switch within the field device to switch on the power to the device from a provisioned or redundant power line. The IS guidelines prohibit internal power switching because field devices are often installed in proximity to volatile substances or volatile processes, and thus there is higher possibility of causing an explosion by arcing or generating sparks when a voltage or a power connection is applied to the field device. For reference, an internal switch may be considered any switch that is integrally connected within or physically housed within a field device and/or that is fixed to the field device.
Related IS guidelines also advise against switching on power within a PTD that is connected to a field device and that is located within a vicinity of the field device. IS standards generally require manual intervention when applying power to a non-operating or non-powered field device installed in the field. Though it may be desirable to configure existing PTDs with automatic power functions for powering a field device, this configuration is generally prohibited under the IS standards.
To comply with IS standards, some existing PTDs include an interface with four connection ports for coupling four lines or wires between the PTD and a field device undergoing testing. Generally, a first pair of lines is used for transmitting communication signals at a first voltage range and a second pair of lines is used for powering the field device at a second and higher voltage or voltage range. The first pair of lines is primarily used whenever the field device is undergoing testing, and the second pair of lines/wires is used only when power is needed to be provided to the field device to enable the field device to execute a function (e.g., a test function or a configuration function) by the field device. In this manner, additional power to the field device undergoing testing always requires manual intervention that includes connecting additional wires between the field device and the PTD. In short, IS standards have generally limited the development of portable field device testing equipment to require two separate sets of lines or lead sets and three or four ports for connecting a field device to the portable testing equipment. While it is common practice to carry two lead sets with a PTD, this activity may not be the most efficient or convenient solution for field technicians when performing maintenance functions.